Synthesis, structural characterization, and antibacterial activity of Cu(II) and Zn(II) complexes with tetradentate Schiff bases

Article abstract of DOI:10.1080/15533174.2014.963243

Biological activity study on Cu(II) and Zn(II) complexes is a challenging issue because of the activity of ligands could be modulated by complexation. A new dinuclear copper(II) complex, [Cu₂(L¹)₂]·2CH₃OH (1), a

Full text of DOI:10.1080/15533174.2014.963243

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Synthesis, Structural Characterization and
Antibacterial Activity of Cu(II) and Zn(II) Complexes
with Tetradentate Schiff Bases
Huan-Yu Liua
a School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University,
Zhongshan 528453, P. R. China.
Accepted author version posted online: 18 Feb 2015.
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To cite this article: Huan-Yu Liu (2015): Synthesis, Structural Characterization and Antibacterial Activity of Cu(II) and Zn(II)
Complexes with Tetradentate Schiff Bases, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry,
DOI: 10.1080/15533174.2014.963243
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Synthesis, Structural Characterization and Antibacterial Activity of Cu(II) and Zn(II)
Complexes with Tetradentate Schiff Bases
Huan-Yu Liu
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University,
Zhongshan 528453, P. R. China.
E-mail: liuhuanyu03@163.com
Biological activity study on Cu(II) and Zn(II) complexes is a challenging issue because of
the activity of ligands could be modulated by complexation. A new dinuclear copper(II)
complex, [Cu2(L1)2]·2CH3OH (1), and a new mononuclear zinc(II) complex,
[ZnL2(CH3OH)] (2), where L1 is the dianionic form of the bis-Schiff base N,N’-bis(2-
hydroxy-5-methoxybenzylidene)-1,2-ethanediamine (H2L1), and L2 is the dianionic form of
N,N’-bis(5-fluoro-2-hydroxybenzylidene)-1,2-ethanediamine (H2L2), have been synthesized
and characterized by single-crystal X-ray diffraction. X-ray analysis indicates that the
metal atoms in the complexes are in square pyramidal coordination. Antibacterial activity
of the complexes against Staphylococcus aureus, Escherichia coli, Klebsielle pneumoniae
and Candida albicans strains was studied and compared with that of the free Schiff bases.
Keywords Schiff base, Cu(II) complex, Zn(II) complex, crystal structure, antibacterial activity
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INTRODUCTION
Schiff bases have received much attention because of their versatile coordination ability[1–4]
and interesting biological activities.[5–8] The chemistry of transition metal complexes of Schiff
bases became largely attractive because of their broad profile of pharmacological activity
affording a diverse variety of compounds with different activities.[9–11] Cu(II) and Zn(II)
complexes have been proved to possess excellent biological activities.[12–15] In addition, the role
played by copper and zinc ions in the active sites of a large number of metalloproteins has
stimulated efforts to design and characterize such complexes as models for a better
understanding of biological systems. In this paper, two new Cu(II) and Zn(II) complexes,
[Cu2(L1)2]·2CH3OH (1) and [ZnL2(CH3OH)] (2), where L1 is the dianionic form of the bis-Schiff
base N,N’-bis(2-hydroxy-5-methoxybenzylidene)-1,2-ethanediamine (H2L1), and L2 is the
dianionic form of N,N’-bis(5-fluoro-2-hydroxybenzylidene)-1,2-ethanediamine (H2L2) (Scheme
1), were prepared and characterized. The Schiff bases and the complexes were tested for their in
vitro antibacterial effects against four microorganisms.
EXPERIMENTAL
General Methods and Materials
Copper perchlorate, zinc perchlorate, 5-methoxysalicylaldehyde, 5-fluorosalicylaldehyde and
ethane-1,2-diamine were obtained from Sigma-Aldrich. All other reagents were of analytical
reagent grade. Elemental analyses (C, H, N) were performed with a PE-2400 II apparatus.
Infrared spectra were recorded on KBr pellets with a Nicolet Nexus 670 FT-IR spectrometer in
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the 400–4000 cm–1 range. Molar conductance was measured with a Shanghai DDS-11A
conductometer. X-ray diffraction was carried out on a Bruker SMART 1000 CCD diffractometer.
Caution: Copper perchlorate and zinc perchlorate are potentially explosive, only small
quantity should be used and handled with great care.
Synthesis of the Schiff Bases
To the methanolic solution (30 mL) of salicylaldehyde derivatives (2.0 mmol) were added
methanolic solution (30 mL) of ethane-1,2-diamine (1.0 mmol) with stirring. The mixtures were
stirred for 30 min at room temperature to give yellow precipitation. The solvent was evaporated
to give yellow crystalline products, which were dried by P2O5. For H2L1: Yield 95%. Anal.:
Calcd. for C18H20N2O4 (%): C, 65.84; H, 6.14; N, 8.53. Found (%): C, 65.67; H, 6.23; N, 8.61.
Infrared (IR) (KBr, νmax/cm–1): 3317 (m), 1651 (s), 1253 (m). For H2L2: Yield 91%. Anal.:
Calcd. for C16H16F2N2O2 (%):C, 62.74; H, 5.27; N, 9.15. Found (%): C, 62.89; H, 5.35; N, 9.06.
Infrared (IR) (KBr, νmax/cm–1): 3335 (m), 1655 (s), 1246 (m).
Synthesis of [CuL1]·CH3OH
H2L1 (65.6 mg, 0.2 mmol) and copper perchlorate (75.0 mg, 0.2 mmol) were mixed in
methanol (30 mL). The mixture was refluxed for 2 h and then cooled to room temperature, and
filtered. The filtrate was kept at ambient temperature. Single crystals of the complex, suitable for
X-ray diffraction, were grown from the filtrate upon slow evaporation within a few days. The
crystals were isolated by filtration, washed with methanol and dried in air. Yield 43%. Anal.:
Calcd. for C38H44Cu2N4O10 (%): C, 54.08; H, 5.26; N, 6.64. Found (%): C, 54.23; H, 5.33; N,
6.54. Infrared (IR) (KBr, νmax/cm–1): 1647 (s), 1185 (m).
Synthesis of [ZnL2(CH3OH)]
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H2L2 (61.2 mg, 0.2 mmol) and zinc perchlorate (77.0 mg, 0.2 mmol) were mixed in methanol
(30 mL). The mixture was refluxed for 2 h and then cooled to room temperature, and filtered.
The filtrate was kept at ambient temperature. Single crystals of the complex, suitable for X-ray
diffraction, were grown from the filtrate upon slow evaporation within a few days. The crystals
were isolated by filtration, washed with methanol and dried in air. Yield 71%. Anal.: Calcd. for
C17H16F2N2O3Zn (%): C, 51.08; H, 4.03; N, 7.01. Found (%): C, 50.96; H, 4.10; N, 7.08. Infrared
(IR) (KBr, νmax/cm–1): 1642 (s), 1189 (m).
X-ray Crystallography
The program SAINT was used for integration of the diffraction profiles.[16] Structures were
solved by direct methods using the SHELXS program of the SHELXTL package and refined by
full-matrix least-squares methods with SHELXL (semi-empirical absorption corrections were
applied using the SADABS program).[17, 18] The positions of the non-hydrogen atoms were
located in difference Fourier syntheses and least-squares refinement cycles, and finally refined
anisotropically. H3 atom of complex 2 was located from the electronic density map and refined
isotropically, with O–H distance restrained to 0.85(1) Å. The remaining hydrogen atoms of the
complexes were placed theoretically onto the specific atoms and refined isotropically as riding
atoms. Crystallographic data and experimental details for structural analyses are summarized in
Table 1. Selected bond lengths and angles for the complexes are listed in Table 2.
Antibacterial Assay
The complexes and ligands were tested for their in vitro antibacterial activity against
Staphylococcus aureus, Escherichia coli, Klebsielle pneumoniae and Candida albicans strains
using the paper disc diffusion method (for the qualitative determination) and the serial dilutions
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in liquid broth method (for determination of MIC).[19] Suspensions in sterile peptone water from
24 h cultures of microorganisms were adjusted to 0.5 McFarland. Muller–Hinton Petri dishes of
90 mm were inoculated using these suspensions. Paper disks (6 mm in diameter) containing 10
μL of the substance to be tested (at a concentration of 2048 μg/mL in DMSO) were placed in a
circular pattern in each inoculated plate. Incubation of the plates was done at 37 ºC for 18–24 h.
Reading of the results was done by measuring the diameters of the inhibition zones generated by
the tested substances. Tetracycline and fluconazole were used as a reference substance.
Determination of MIC was done using the serial dilutions in liquid broth method. The
materials used were 96-well plates, suspensions of microorganism (0.5 McFarland), Muller–
Hinton broth (Merck) and stock solutions of each substance to be tested (2048 μg/mL in DMSO).
The following concentrations of the substances to be tested were obtained in the 96-well plates:
1024, 512, 256, 128, 64, 32, 16, 8.0, 4.0 and 2.0 μg/mL. After incubation at 37 ºC for 18–24 h,
the MIC for each tested substance was determined by macroscopic observation of microbial
growth. It corresponds to the well with the lowest concentration of the tested substance where
microbial growth was clearly inhibited.
RESULTS AND DISCUSSION
Synthesis and Characterization
Complexes 1 and 2 were synthesized by the reaction of the Schiff base ligands with copper
perchlorate and zinc perchlorate, respectively, in methanol (Scheme 2). They are soluble in
acetonitrile, methanol, ethanol, DMF and DMSO. Elemental analyses of the complexes are in
accordance with the molecular structures proposed by the X-ray analysis. Both complexes are
stable in air at room temperature. The molar conductivity values in acetonitrile in 23-29 Ω–1 cm2
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mol–1 range indicated that they are non-electrolytes.[20]
Structure Description of Complex 1
The molecular structure of complex 1 is shown in Figure 1. The complex bears
crystallographic inversion center symmetry. The inversion center is located at the midpoint of the
two Cu atoms. Besides, there are two methanol molecules which are connected to the dinuclear
complex molecule via O5–H5···O1 hydrogen bonds [O5–H5 = 0.82 Å, H5···O1 = 1.96 Å,
O5···O1 = 2.762(6) Å, O5–H5···O1 = 166.0º]. The Cu atom is coordinated by two phenolate O
and two imino N atoms of L1 ligand, and by one phenolate O atom of the symmetry related L1
ligand, forming a square pyramidal geometry. The Cu-O/N bonds in the basal plane of the square
pyramidal geometry are within normal values and comparable to those observed in similar
copper complexes.[21,22] It is clear that the apical Cu-O bond is much longer than the basal ones,
indicating the coordination of the O atom to the Cu center is not very strong. The cis and trans
angles in the basal plane vary between 84.83(15) and 92.93(14)º, and 175.80(14) and
176.92(14)º. The dihedral angle between the two benzene rings is 3.3(3)º.
Structure Description of Complex 2
The molecular structure of complex 2 is shown in Figure 2. It is a mononuclear zinc complex.
The Zn atom is coordinated by two phenolate O and two imino N atoms of L1 ligand (at the basal
position), and by a methanol O atom (at the apical position). The Zn atom deviates from the
least-squares plane defined by the four basal donor atoms by 0.427(2) Å. The Zn-O/N bonds are
within normal values and comparable to those observed in similar copper complexes.[23,24] The
cis and trans angles vary between 78.61(7) and 96.61(6)º, and 139.25(7) and 166.66(7)º. The
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dihedral angle between the two benzene rings is 28.6(3)º. In the crystal structure of the complex,
two molecules are linked via O3–H3A···O2 hydrogen bonds [O3–H3A = 0.83(1) Å, H3A···O2i
= 1.83(1) Å, O3···O2i = 2.658(2) Å, O3–H3A···O2i = 174(3)º, symmetry code for i: 1 – x, – y, –
z], to form a dimer (Figure 3).
Infrared Spectra
Infrared spectra provide valuable information regarding the functional group attached to the
metal center. The IR spectra of the free Schiff bases and the complexes exhibit broad bands in
the range 3300–3500 cm-1 due to the O–H stretching. Weak bands in the region 2800–3050 cm-1
are related to C–H stretches. The spectra of the free Schiff bases show phenolic C–O stretching
vibrations at 1275–1283 cm-1, while in the complexes they shifted to lower frequencies.
Vibrations of the azomethine groups of the free Schiff bases are observed at 1632 cm-1 for H2L1
and 1636 cm-1 for H2L2. In the complexes, these bands are shifted to higher frequencies. This
suggests the coordination of the azomethine nitrogen to the metal atoms. New bands in the
region 400–570 cm-1, which are absent in the free Schiff bases, may be attributed to M–O and
M–N vibrations.
Antibacterial Activity
The free Schiff bases and the complexes were screened for their antibacterial activities. The
results are listed in Table 3. The Schiff base H2L2 has stronger activity than H2L1, which
indicated that the F atom contributes to the biological processes. A comparison of the
antibacterial activity of the copper and zinc complexes with the corresponding free Schiff bases
has shown that the complexation of Schiff bases with metal atoms enhances their antibacterial
activity manifold in general. Composition of coordination site and the geometry of the tested
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complexes seem to be the principal factor which influences the antibacterial activity. Yet, all the
compounds have lower activity than the control drugs. The trends of this work are in accordance
with those reported in the literature that metal complexes are usually more effective antibacterial
materials than the corresponding ligands.[25,26]
From the above results, it can be seen that the chelation may reduce the polarity of the metal
ion because of partial sharing of its positive charge with the donor group and possible electron
delocalization over the whole chelate ring. Also, we can consider that the coordination may
facilitate the ability of a complex to cross the lipid layer of the bacterial cell membrane and in
this way may be affected the mechanisms of growth and development of microorganisms.[27]
SUPPLEMENTARY DATA
CCDC 940554 (1) and 972038 (2) contain the supplementary crystallographic data for this
paper.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
ACKNOWLEDGMENTS
The work was supported by the Guangdong Pharmaceutical University Young Teachers' Fund
and the Ten Hundred Thousand Project of the Bureau of Education of Guangdong Province, P.R.
China.
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TABLE 1
Crystallographic information for the complexes
1
C38H44Cu2N4O10
843.8
2
C17H16F2N2O3Zn
399.7
Formula
Formula weight
Crystal shape/color
T (K)
Block/blue
298(2)
Block/colorless
298(2)
Crystal
(mm3)
dimensions
0.32 × 0.30 × 0.27
0.23 × 0.23 × 0.22
Crystal system
Space group
a (Å)
Monoclinic
P21/n
Triclinic
P-1
11.111(1)
7.5618(7)
21.655(1)
90
8.8701(8)
9.6530(9)
10.5033(9)
114.563(3)
91.742(3)
b (Å)
c (Å)
α (º)
92.356(2)
β (º)
γ (º)
92.356(2)
1817.8(3)
2
97.006(3)
808.6(1)
2
V (Å3)
Z
Dcalc (g cm–3)
µ (Mo Kα) (mm-1)
1.542
1.642
1.560
1.235
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F(000)
876
408
Measured reflections
Unique reflections
15490
3379
8251
3298
2815
Observed reflections (I 2557
≥ 2σ(I))
Min.
and
max. 0.6933 and 0.7315
0.7156 and 0.7254
transmission
Parameters
Restraints
248
0
231
1
Goodness of fit on F2
R1, wR2 [I ≥ 2σ(I)]a
R1, wR2 (all data)a
1.063
1.047
0.0560, 0.1406
0.0322, 0.0750
0.0427, 0.0795
0.0804, 0.1544
2
2 2 1/2
a R1 = Fo – Fc/Fo, wR2 = [∑ w(Fo – Fc2)/∑ w(Fo ) ]
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TABLE 2.
Selected bond lengths (Å) and angles (º) for the complexes with estimated standard deviations
(e.s.d.s) in parentheses
1
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–O(2A)
1.902(3)
1.944(4)
2.833(5)
Cu(1)–O(2)
Cu(1)–N(2)
1.903(3)
1.948(4)
O(1)–Cu(1)–O(2) 89.70(13)
O(2)–Cu(1)–N(1) 176.92(14)
O(2)–Cu(1)–N(2) 92.43(14)
N(1)–Cu(1)–O(2A) 86.61(15)
O(1)–Cu(1)–O(2A) 98.34(16)
2
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
92.93(14)
175.80(14)
84.84(15)
N(2)–Cu(1)–O(2A) 85.10(15)
O(2)–Cu(1)–O(2A) 94.60(16)
Zn(1)–O(1)
Zn(1)–O(3)
Zn(1)–N(2)
1.9607(16)
2.0756(15)
2.0827(18)
Zn(1)–O(2)
Zn(1)–N(1)
1.9903(14)
2.0889(18)
O(1)–Zn(1)–O(2) 96.61(6)
O(2)–Zn(1)–O(3) 108.65(6)
O(2)–Zn(1)–N(2) 89.09(6)
O(1)–Zn(1)–N(1) 89.30(7)
O(3)–Zn(1)–N(1) 111.35(7)
O(1)–Zn(1)–O(3)
O(1)–Zn(1)–N(2)
O(3)–Zn(1)–N(2)
O(2)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
92.33(7)
166.66(7)
97.27(7)
139.25(7)
78.61(7)
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TABLE 3
Antibacterial activity as MIC values (μg/mL)
Compound
Staphylococcus Escherichia
Klebsielle
Candida
albicans
512
aureus
64
coli
128
32
pneumoniae
H2L1
64
16
8.0
4.0
1.0
-
H2L2
32
256
2.0
4.0
0.25
-
8.0
8.0
2.0
-
128
1
128
2
Tetracycline
Fluconazol
-
2.0
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FIG. 1. A perspective view of complex 1 with the atom labeling scheme. Thermal ellipsoids are
drawn at the 30% probability level.
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FIG. 2. A perspective view of complex 2 with the atom labeling scheme. Thermal ellipsoids are
drawn at the 30% probability level.
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FIG. 3. Molecular packing structure of complex 2, viewed along the z axis. Hydrogen bonds are
indicated by dashed lines.
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Scheme 1. H2L (X = OMe for H2L1; X = F for H2L2)
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N
O
N
O
Cu
X
X
X
O
O
N
X
Cu(ClO4)2
Cu
N
N
N
2CH3OH
X
OH HO
X
Zn(ClO4)2
N
O
N
Zn
X
O
X
OMe
H
Scheme 2. The synthesis of the complexes (X = OMe for 1; X = F for 2)
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